Microfilm is a common form of information storage comprising the photographic reduction of paper documents on to film. The information stored on the microfilm can be retrieved by projecting light through the microfilm, enlarging the image on the film with an optic system, and exposing a photo sensitive surface to the enlarged image, for development of an enlarged copy of the image recorded on the microfilm. The use of microfilm allows for the storage of large amounts of information in a physically small space on a durable yet inexpensive medium.
Microfilm aperture cards comprise standard computer punch cards having an aperture window in the card and a microfilm frame positioned across the aperture. The card can include punched data relating to the identification and indexing of the microfilm frame. The aperture cards can be fed into an aperture card printer where the cards are selected one by one into a scanning station. Light is projected through the microfilm frame at the scanning station to form an enlarged image of the microfilm onto a photo sensitive surface, from which a full scale paper print of the information stored on the microfilm frame can be generated by conventional photographic or electrophotographic techniques.
Microfilm aperture cards are particularly well adapted for use in recording and storing engineering documents. Engineering applications often require large print size, operation in adverse environments and demand for output on vellum and offset masters as well as plain paper. The 35 mm format and predominant use of negative film in microfilm aperture cards accommodate the special demands of engineering documentation.
The quality of the print produced by a microfilm aperture card printer depends, among other things, upon the relationship between the intensity of the light projected onto the microfilm and the density (darkness) of the microfilm. The intensity of the light can be controlled by adjusting controls on the printer. The density of the film, however, is a function of the type of microfilm used, the development process used to create the film, and the amount of information on an individual frame. Accordingly, print quality can often be improved by measuring the density of a microfilm frame just prior to scanning the frame for printing, and adjusting the intensity or duration of the light projected through the frame as a function of the frame's density. Even when using printing processes known for having exceptional print latitude at conventional speeds (the ability to accommodate wide variances in film density), adjusting the exposure of the card as a function of film density may be desirable when faster throughput is desired.
While devices for automatically controlling the exposure of a microfilm frame as a function of microfilm frame density are known, adaptation of such devices to microfilm aperture card printers presents several special problems. For instance, the environment in which aperture card printers are operated in is frequently hostile in terms of ambient light conditions and airborne dust, and frequent calibration of an installed exposure control device would be desirable. Moreover, the scanning stations of conventional aperture card printers have little extra space for installation of additional equipment.
Further problems are presented by the fact that conventional microfilm density measurement devices often employ mechanically moved mirrors and light shields so that light intensity can be measured in the actual path of travel the light will take during the exposure step, without blocking the light during the exposure step. Aperture card printers, however, are designed to automatically and rapidly print large numbers of cards in succession, and the constant and rapid positioning and repositioning of mechanical elements is not acceptable. Another problem in adapting some of the available conventional density measurement devices to aperture card readers is presented by the fact that the image density can vary widely on a particular aperture card; A density measurement at one portion may therefore not be representative of the measurement at a second portion. A density measurement device for use with an aperture card printer must somehow accommodate for such variances in film density.
An automatic exposure control device that could adjust the exposure of a microfilm frame as a function of the frame's density, which was particularly adapted for use in microfilm aperture card printers, but could also be used in other microfilm printing applications, would be a decided improvement over conventional exposure control devices.